function ListNode(val) {
    this.val = val;
    this.next = null;

}
const node = new ListNode(1)
const node2 = new ListNode(2)
const node3 = new ListNode(3)

node.next = node2
node2.next = node3

node.next = node3//断开关于node2的链接，将1链接到3
